Prevalence of Overweight and Obesity and Their Associations with Unhealthy Eating Behaviour, Physical Inactivity, and Sedentary Lifestyle among Secondary-School Adolescents in Derna, Libya: A Comparative Study by School Type and Sex

Background: Adolescent overweight and obesity represent major global public health challenges due to their increasing prevalence and long-term health consequences. Although unhealthy eating behaviour, physical inactivity, and sedentary lifestyle are established risk factors, evidence from Libya remains limited, particularly among secondary-school adolescents. This study aimed to determine the prevalence of overweight and obesity among adolescents in Derna, Eastern Libya, and to examine their associations with behavioural risk factors, with comparisons by school type and sex. Methods: A school-based cross-sectional study was conducted among 4,500 adolescents aged 15–19 years using multistage cluster sampling from governmental and private secondary schools. Data were collected using a structured questionnaire adapted from the WHO STEPwise approach, with demonstrated reliability (Cronbach’s α = 0.79–0.91). Anthropometric measurements, including BMI, waist circumference, body fat, and visceral fat, were obtained using standardised procedures. Weight status was classified according to WHO BMI-for-age reference. Statistical analyses included descriptive statistics, chi-square tests, t-tests, and multivariable logistic regression using SPSS, with significance set at p < 0.05. Results: The overall prevalence of overweight and obesity was 27.8% (95% CI: 26.5–29.1%), including 18.4% overweight and 9.4% obesity. Boys had a significantly higher prevalence than girls (31.1% vs. 24.5%, p < 0.001), and private-school students showed higher prevalence than governmental-school students (31.8% vs. 25.8%, p < 0.001). Unhealthy eating behaviours, particularly sugar-sweetened beverage consumption (OR = 1.72) and fast-food intake (OR = 1.64), were significantly associated with overweight/obesity. Physical inactivity (OR = 1.62) and prolonged mobile phone use (OR = 1.84) were also significant predictors (p < 0.001). All hypothesised associations were supported. Conclusion: Overweight and obesity are highly prevalent among adolescents in Derna and are strongly associated with modifiable behavioural factors. Differences by sex and school type highlight the influence of contextual determinants. These findings support the need for targeted, school-based and community interventions to address adolescent obesity.